Optical projection systems, such as televisions and computer monitors, use cathode ray tubes (CRTs) as displays. A liquid crystal on silicon, or LCOS, light modulator, is an alternative display component that has some advantages over CRTs. In particular, LCOS light modulators are flat, thus occupying less space, and use less power than CRTs.
LCOS displays or panels consist of layered components that form an array of individual pixels, typically numbering a million or more. A transparent surface layer of glass or plastic substrate is disposed over a middle layer of liquid crystal material, which is further supported by an underlying layer of silicon substrate, also known as a backplane. The transparent layer supports transparent electrodes on its inward surface, which are typically formed from indium tin oxide (ITO). Across the liquid crystal, metal electrodes are disposed on the silicon backplane. The metallic backplane serves both electrical and optical functions. These electrodes are patterned, with a reflecting mirror, or micro-mirror, allocated for each pixel.
When a voltage is applied across the electrodes of an individual pixel, the liquid crystal material therein may change, producing a refractive index response. When the panel is illuminated with polarized light, the refractive index can be used to form a pixel's display intensity. The intensity of the light that is ultimately displayed is thus modulated by the voltage supplied to the panel, and this modulation takes place independently at each pixel. The voltage supplied to the LCOS pixels may be analog or digital.
Particularly for LCOS projection systems, a high-intensity, stable light source is typically used to illuminate the LCOS panel. Some LCOS projection systems include ultra-high pressure (UHP) arc lamps to provide the high-intensity light. UHP arc lamps also consist of two electrodes, in this case, embedded in a gas medium. When the lamp receives power, an electrical arc is generated between the two electrodes, producing the high-intensity light.
The arc, an ionized gas or plasma formation within the arc lamp, is not always stable. Sometimes, the plasma medium will form into a ball that is near one of the electrodes. In other cases, the ball randomly jumps between the electrodes, causing the resulting light to flicker. The gap width (electrode separation) of the arc may widen over time. Since the gap width affects overall display brightness, keeping the arc stable is highly desirable for bright display products.
One way to improve the stability of a UHP arc lamp is to supply the lamp with a short, periodic change in current, or pulsed over-drive current, instead of a continuous current. The pulsed over-drive current, or overdrive pulse, stabilizes the arc in the lamp, known herein as a pulse stabilized arc lamp. However, a temporary and periodic increase in the lamp intensity also occurs.
Some LCOS panels are supplied with pulse width modulated (PWM) signals. For a PWM LCOS projection system that uses a pulse stabilized arc lamp, the periodic increase in lamp intensity produces a noticeable and objectionable variation, or flicker, in the displayed image, due to the beat frequency between the image update or refresh rate and the stabilization pulse rate in the LCOS display. The lamp phenomenon may also produce tone scale corruption in the display. Overdrive pulses generally do not occur in all PWM cycles. As a result, with no correction, there is a perceptual brightness difference between tones formed in PWM cycles that receive an overdrive pulse and tones formed in non-overdrive pulse PWM cycles. A recalculation of the PWM duty cycle may re-adjust the tone. Other display technologies, such as micromirror-based projectors, which also use PWM, may experience similar problems with pulse stabilized arc lamps.
Thus, there is a continuing need for a way to use a pulse stabilized arc lamp in a PWM-based display with reduced image flicker.